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Volume 6 Supplement 3

AIDS Vaccine 2009

P19-03. Molecular mechanisms for enhancing the antigenicity of the carbohydrate epitope of the broadly neutralizing anti-HIV-1 antibody 2G12

Background

The broadly neutralizing antibody 2G12 is uniquely capable of recognizing the self-carbohydrate shield on the HIV-1 envelope glycoprotein gp120 and escaping immune tolerance. The preferred substrate of 2G12 is the Man4(D1)-arm of high-mannose glycans clustered on the gp120 outer surface. Remarkably, 2G12 has been shown to exhibit higher affinity for the non-self monosaccharide D-fructose than for D-mannose. D-fructose, in its pyranose form, resembles D-mannose, but with different substitutions at the anomeric and C-5 positions. These observations suggested other 'non-self' modifications to D-mannose may also enhance 2G12 binding. We consequently found that D-mannose with a C-6 methyl substitution binds 2G12 more strongly than both D-mannose and D-fructose. Moreover, introduction of this non-self modification to the terminus of the D1-arm creates the most potent monovalent 2G12 ligand known. We uncover here the molecular basis of this enhanced antigenicity, and the higher affinity of 2G12 for D-fructose over D-mannose, through high resolution crystallographic analyses.

Methods

Crystal structures of 2G12 in complex with the modified D-mannose and D1-arm, and D-fructose were determined to 1.75, 2.85, and 1.95 Å, respectively.

Results

The crystal structures of 2G12 in complex with the modified D-mannose and D1-arm revealed the C-6 methyl substitution enhances antigenicity by making additional hydrophobic contacts with the aromatic side chain of TyrL94. The D1-arm mimic otherwise adopts the same conformation as the natural D1-arm in complex with 2G12. The complex with D-fructose revealed the non-self monosaccharide is indeed bound in its pyranose form and its enhanced antigenicity is due to several additional H-bonds mediated by its C-5 hydroxyl.

Conclusion

The crystal structures reported here uncover different mechanisms for creating potent non-self monosaccharide antigens of 2G12. The fact the D1-arm mimic maintains the same interactions with 2G12 as the natural D1-arm is encouraging in terms of its potential as an immunogenic fragment to elicit a 2G12-like immune response.

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Correspondence to Z Fulton.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Fulton, Z., Doores, K., Scanlan, C. et al. P19-03. Molecular mechanisms for enhancing the antigenicity of the carbohydrate epitope of the broadly neutralizing anti-HIV-1 antibody 2G12. Retrovirology 6, P323 (2009). https://doi.org/10.1186/1742-4690-6-S3-P323

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Keywords

  • Monosaccharide
  • Prefer Substrate
  • Immune Tolerance
  • Envelope Glycoprotein
  • Crystallographic Analysis